Molybdenum trioxide (MoO₃) as a cathode buffer layer is inserted between LiF and Al to improve the efficiency of white organic light-emitting diodes (OLEDs) in this paper. By changing the MoO₃ thickness, a higher current efficiency of 5.79 cd/A is obtained at a current density of 160 mA/cm² for the device with a 0.8 nm-thick MoO3 layer as the cathode buffer layer, which is approximately two times greater than that of the device without MoO₃. The mechanism for improving the device efficiency is discussed. Moreover, at a voltage of 13 V, the device with a 0.8 nm-thick MoO₃ layer achieves a higher luminance of 22370 cd/m², and the Commission Internationale de I’Eclairage (CIE) color coordinate of the device with 1 nm-thick MoO₃ layer is (0.33, 0.34), which shows the best color purity. Simple electron-only devices are tested to confirm the impact of the MoO₃ layer on the carrier injection.